1. Field of the Invention
The present invention relates to an improvement in a blank guide device for guiding cylindrical blanks employed to produce can bodies to the welding point between electrode rolls of a resistance welding machine.
2. Description of the Prior Art
To produce can bodies by seam welding, it is general practice to weld the overlapped edges of a cylindrical blank which is obtained by rolling a rectangular sheet material. In such a manufacturing process, a tool known as a Z-bar is generally employed to guide cylindrical blanks when they are fed to the welding point between the electrode rolls and to maintain the overlapping of the edges of the blanks at a constant width when they are welded.
The above-described manufacturing method will be explained hereinunder with reference to the accompanying drawings. Referring first to FIGS. 1A-1C which show the process of manufacturing can bodies, a rectangular sheet material shown in FIG. 1A is rolled into a cylindrical shape, such as that shown in FIG. 1B, and is then fed to the welding point between the electrode rolls while being guided by a Z-bar 4 in the manner shown in FIG. 2. In this case, both edges 2 and 3 of the cylindrical blank 1 are respectively pressed by upper and lower electrode rolls A and B so that the overlapping of the edges 2 and 3 is maintained at a constant width by means of the Z-bar 4 in the manner shown in FIG. 3 which is a cross-sectional view of the Z-bar 4, whereby the cylindrical blank 1 is seam-welded to form a cylindrical can body 1', such as that shown in FIG. 1C.
FIG. 4A is a side view of the Z-bar 4, FIG. 4B is a front view of the Z-bar 4, and FIG. 4C shows a worn portion 7' of a guide surface 7. As will be clear from these Figures, the bottom surface 5 of a groove 6 in the Z-bar 4 which guides the edge 3 of the cylindrical blank 1 and the bottom surface 7 of a groove 8 which guides the other edge 2 are continuously press-contacted by the edges 2 and 3 of the cylindrical blank 1 which slide thereon. For this reason, the bottom surfaces 5, 7 quickly become worn.
When the respective bottom surfaces 5 and 7 of the grooves 6 and 8 of the Z-bar 4 have become worn, the overlapping of the edges 2 and 3 of the cylindrical blank 1 varies and, therefore, if the wear exceeds an allowable amount, it is necessary to suspend the production. Not only when both the bottom surfaces 5 and 7 have become worn but also when only one of them has become worn, if the wear exceeds an allowable amount, the weld line varies, so that it is not possible to obtain an excellent welded seam. For this reason, even if the wear of the other bottom surface has not yet exceeded the allowable amount, such a Z-bar needs to be replaced with a new one.
As shown in FIG. 4C, the wear of the Z-bar 4 generally concentrates on the guide outlet side at its rearmost end, and the Z-bar 4 currently used in practice is constituted by an integral member, which is continuous as a whole. For this reason, with the conventional Z-bar 4, when the wear of a portion at its rearmost end exceeds the allowable amount, it is necessary to replace the whole of the Z-bar 4. The conventional Z-bar 4 has a complicated structure, which is difficult to machine; consequently, it is expensive--about 1,000 dollars per Z-bar. Moreover, the Z-bar 4 is mounted on a welding machine with a complex array of surrounding components. Therefore, replacing the Z-bar 4 requires many steps of disassemble and reassemble the surrounding parts, and it also takes much time to adjust a new Z-bar after it has been mounted. A typical Z-bar requires seven to eight hours to replace, an obviously undesirable characteristic.